Interesting - Queen sacrifice to smother the King so knight can checkmate from, f7. The classic version of this is with the black King smothered in the h8 corner. I did not see this sequence starting out, but the first two moves looked like the best available, and then it became apparent what could follow.

Actually, the second move was the hardest. In a game I would be focusing on trying to get in a position where I could fork the king and queen with my knight. But that double check is so inviting, I couldn't resist playing it, especially knowing it was a mate-in-4 and the black Queen was only relevant in that she had my black-squares Bishop pinned.============EDIT==============The real advantage of 2.Nd6++ is that it leaves the knight covering e8, so the King can't capture the white Queen on that square. But I didn't see that when I played it; I just saw that it was a lovely double check. Only after I played it did I realize that now I could smother the black King with a Queen sacrifice at e8 (and the Knight could checkmate on the next move).